According to the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard, two modes are available as communication modes used when a plurality of wireless terminals communicate with each other. One is an infrastructure mode in which the wireless terminals communicate with each other via a base station (access point, to be referred to as an AP hereinafter). The other is an ad hoc mode in which the wireless terminals directly communicate with each other without mediacy of a specific AP.
Also, as the operation states of a wireless terminal based on IEEE802.11 standard, two operation modes are available. One is an “active mode” representing a normal operation, and the other is a “power save mode (to be referred to as a PS mode hereinafter)” representing a power saving mode operation. The PS mode has two states, i.e., an “Awake state (receivable state)” in which power is completely supplied to a transmission/reception circuit and the like in the wireless unit of the wireless terminal, and a “Doze state (inactive state)” in which only minimum power is supplied to that. In the “Awake state”, data can be transmitted/received. However, in the “Doze state”, no data can be transmitted/received. In the PS mode, the wireless terminal repeatedly shifts between the “Awake state” and “Doze state”, periodically.
In the ad hoc mode, in order to transmit data to the wireless terminal in the PS mode, an ATIM (Announcement Traffic Indication Message) frame is transmitted into an ATIM window. The ATIM window is a period which starts from TBTT (Target Beacon Transmission Time) of the periods shown in FIG. 13, and its size can be known in accordance with a “Beacon”, “Probe Response”, and the like. In this period, all the terminals including the terminals in the PS mode are in the “Awake state”, and only the “Beacon” and ATIM frame are allowed to be transmitted.
A PS mode operation in the ad hoc mode will be described below with reference to FIG. 13. In an example shown in FIG. 13, three wireless terminals, i.e., terminals A, B, and C construct a network in the ad hoc mode, and operate in the PS mode.
In order to transmit data from the terminal A to the terminal B, first, the terminal A announces data transmission by sending the ATIM frame to the terminal B. Upon reception of the ATIM frame, the terminal B recognizes that the data is to be transmitted from the terminal A. Accordingly, in a “Beacon” interval, the terminal B does not shift from the “Awake state” to the “Doze state” but is maintained in the “Awake state”. Then, in order to notify the terminal A of reception of the ATIM frame from the terminal A, the terminal B transmits an ACK (acknowledgement) signal to the terminal A. Upon confirmation of reception of the ACK signal, the terminal A transmits the data to the terminal B. When the data is completely received from the terminal A, the terminal B transmits the ACK signal to the terminal A. When the terminal A confirms reception of this ACK signal, one data transmission/reception has been performed between the terminals A and B.
Alternatively, as shown in FIG. 13, a terminal C having no relationship to this data transmission/reception repeatedly shifts between the “Awake state” and “Doze state” unless the ATIM frame is received.
When the terminal B transmits data by multicasting, the ATIM frame must also be transmitted. In this case, all the terminals in the network are in the “Awake state”. However, the ACK signal need not be transmitted to this ATIM frame.
In another power saving management method in the ad hoc mode, it is detected whether a self terminal stores extra power. If it is detected that the self terminal stores extra power, the terminal preferentially outputs the “Beacon” (see Japanese Patent Laid-Open Publication 9-135254 (Japanese Patent No. 2708028)).
However, the IEEE802.11 standard does not define a method of confirming whether a communication partner station functions in the PS mode when the self-station operates in the ad hoc mode. Hence, when the partner station operates in the PS mode, and a self-station is to communicate with the partner station without knowing the partner station state, the self-station can start transmitting the data without the ATIM frame. In this case, since the partner station may be in the “Doze state”, the partner station may fail to receive the data, and normal data transmission is not guaranteed.
The IEEE802.11 standard does not define a method of notifying the partner station of the self-station state when the self-station operates in the PS mode. Hence, when the self-station operates in the PS mode, and the partner station communicates with the self-station without knowing the self-station state, the partner station transmits the data without the ATIM frame. Therefore, when the self-station is in the “Doze state”, the self-station may fail to receive the data, and normal data transmission is not guaranteed.